As used herein, isolate refers to a feature that does not have neighboring geometries that can affect optical proximity effect around. In general if a feature does not have any neighbor within ˜1 um, it can be considered isolated.
As used herein, dense refers to a feature that has neighboring geometries at approximate minimum widths and spaces allowed by a design rule.
Lithographic projection apparatus (tools) can be used, for example, in the manufacture of integrated circuits (ICs). When using the various tools, a mask can be used that contains a circuit pattern corresponding to an individual layer of the IC, and this pattern, usually having many designs, can be imaged onto a target portion (e.g., comprising one or more dies) of a substrate, such as a silicon or other wafer comprising a semiconductor, that has been coated with a layer of radiation-sensitive material, such as a resist. In general, a single wafer may contain a network of adjacent target portions that can be successively irradiated using a projection system of the tool, one at a time.
One of the goals in IC fabrication is to faithfully reproduce the original circuit design on the wafer using the mask or reticle. However, reproduction of the original circuit design is complicated from one IC to the next as the focusing of the lithographic projection apparatus may drift either in a positive or negative direction.
To monitor for focusing errors during the photolithography process, the Critical Dimensions (CDs) of dense and isolate patterns are measured by a Critical Dimension Scanning Electron Microscope (CDSEM) after a group of wafers are patterned on a lithography tool. Isolate patterns are usually more sensitive to the projected image's focus offset, the CDs change very rapidly as the image plane gets away from the optimum position, while dense CDs gradually change through focus offsets, i.e., the dense and isolate patterns have different magnitudes of variation with defocusing. The measured dense/isolate CDs and their differences or bias show if the wafer was patterned with appropriate energy at an optimum image plane.
While the absolute amount of defocus can be estimated by using dense-isolate CD bias analysis, it is difficult to determine if the focus has drifted in a positive or negative direction. Conventionally, to determine a direction of defocus, another exposure/focus matrix wafer is patterned and inspected by CDSEM. Typically this process takes a few hours of engineer resources to complete and creates significant manufacturing delay.
An alternative conventional technique to determine the amount of positive or negative defocus is to use in-situ focus monitoring. In-situ focus monitoring uses a ˜90 degree phase shift pattern on a production reticle to find the focus drift direction, as shown in U.S. Pat. Nos. 5,300,786 and 7,056,625. However, in-situ focus monitoring is very complicated and an expensive addition to a reticle manufacturing process.
Another alternative conventional technique to determine the amount of positive or negative defocus is to use a focus monitor structure, i.e., a series of densely packed lines and an orthogonal line, that is placed on a reticle or mask near the production device structures, such as integrated circuits, to monitor the focal conditions of the lithography process as well as other parameters, such as a critical dimension, and proximity effects, as disclosed by U.S. Pat. No. 6,063,531. By manually or automatically inspecting the focus monitor structure after it is patterned into a layer of resist, including measuring the width of the resist lines and the resist profile angle of the orthogonal line, information relating to the critical dimension as well as the focal conditions of the lithography process can be determined.
Thus, there is a need to overcome these and other problems of the prior art to determine the direction of defocus during a lithography process.